This invention relates to angular orientation indicators and has particular reference to optical devices therefor.
In providing inertial navigation instruments for shipborne navigation systems, a need arises to isolate the inertial measurement unit (IMU) of a system from the ship structure in order to avoid the effects of structure-transmitted shock on the operation of the system. In a recently adopted design, a shockmount provides isolation of the IMU from the ship structure by a six-degree-of-freedom, low-frequency, elastomeric suspension which allows the IMU compliance in both rotation and translation. This suspension concept enables an elegantly simple mechanical suspension design to be used which is much lower in cost and which provides better isolation of the IMU from angular acceleration than does the use of conventional, irrotational ("hard") shockmounts.
In order to use this shockmount concept, it is necessary to maintain precise and continuous measurement of the attitude or angular orientation of the IMU relative to a ship-fixed reference frame so that the ship's roll, pitch and heading can be determined. These are typically required system outputs. Thus, a need arises for apparatus to provide an accurate, direct and continuous measurement of the IMU attitude relative to the ship.
A tracking autocollimator could be used for this purpose. Such a device uses a mirror mounted for rotation on a two-axis gimbal. However, such an approach has the disadvantage of requiring the use of mechanical moving parts in the gimbal structure. As is well known, the use of such mechanical moving parts is to be avoided, where possible, since they decrease the reliability of a device while increasing its complexity and cost.